To reduce switching losses and alleviate electromagnetic interference (EMI), soft switching technique has been developed for power converters. For example, one soft switching converter is an active clamped resonant dc link (ACRDCL) converter in U.S. Pat. No. 4,864,483, issued Sep. 5, 1989, which is hereby incorporated by reference. In the ACRDCL converter, a resonant circuit, incorporated with an active clamping switch and clamping capacitor, is used as an interface between a dc power supply and a dc bus supplying an inverter. The ACRDCL resonates periodically, bringing the dc link voltage to zero once each cycle. The inverter switching devices are switched on and off at zero voltage instants of the resonant dc link, thus achieving essentially lossless switching. However, the ACRDCL converter has some disadvantages, such as, high voltage stress across the inverter switches and continuous resonant operation of the dc link. To overcome the disadvantages of the ACRDCL converter, an auxiliary quasi-resonant dc link (AQRDCL) converter has been developed in U.S. Pat. No. 5,172,309, issued Dec. 15, 1992, which is hereby incorporated by reference. The AQRDCL converter is employed to achieve soft-switching in an inverter coupled to a dc power supply via a resonant dc link circuit. The resonant dc link circuit includes a clamping switch limiting the dc bus voltage across the inverter to the positive rail voltage of the dc supply and auxiliary switching device(s) assisting resonant operation of the resonant bus to zero voltage in order to provide a zero-voltage switching opportunity for the inverter switching devices as the inverter changes state.
Despite their advantages, both the ACRDCL converter and the AQRDCL converter have the following common disadvantages: (1) The resonant dc link circuit acts as an interface (i.e., a dc-to-dc converter) between the dc power supply and the inverter and needs to transmit real power and to carry dc current from the dc power supply to the inverter or from the inverter back to the dc power supply via switch(es) and/or resonant component(s), which can lead to significant power losses; (2) The voltage clamping, voltage control, and charge balancing become difficult due to the real power transmission; (3) The current stress on the auxiliary switch(es) and clamping switch(es) is at least as high as that on the inverter main switches; and (4) Two resonant dc link circuits are needed for an ac-to-dc-to-ac converter to implement soft-switching at both ac-to-dc power conversion stage and dc-to-ac power conversion stage.
Therefore, it is desirable to develop a new soft-switching power converter that can overcome the above-mentioned disadvantages of the ACRDCL converter and AQRDCL converter. Accordingly, an auxiliary resonant dc tank (ARDCT) is employed merely to provide a quasi-resonant or resonant dc bus across the converter without transmitting real power and carrying dc current. Moreover, such an ARDCT circuit should have no problems of voltage clamping and balancing and be capable of providing opportunity for soft-switching at both ac-to-dc power conversion stage and dc-to-ac conversion stage of an ac-to-ac converter, thus making the converter circuit more compact and efficient.